An article that includes a security feature, such as a value document, can be validated in many ways. Some methods involve visible (i.e. overt) security features that are disposed on or incorporated into the article, such as a hologram on a credit card, an embossed image or watermark on a bank note, a security foil, a security ribbon, colored threads or colored fibers within a bank note, or a floating and/or sinking image on a passport. While these features are easy to detect with the eye and may not require equipment for authentication, these overt features are easily identified by a would-be forger and/or counterfeiter. As such, in addition to overt features, hidden (i.e. covert) features may be incorporated in value documents. Examples of covert features include invisible fluorescent fibers, chemically sensitive stains, and luminescent pigments or fluorescent dyes that are incorporated into the substrate of the value document. Covert features may also be included in the ink that is printed onto the substrate of the value document or within the resin used to make films that are used in laminated value documents.
Value documents, such as banknotes, are often processed at high speed in sorting machines such as banknote processing systems, and it is desirable to conduct validation of the value documents in the sorting machines to maximize efficiency and minimize a risk of counterfeit value documents from passing undetected. One covert feature that has been developed to validate value documents that are moving at high speed involves incorporating luminescent phosphors in or on the value documents through paper, inks, or other security features that are employed to make the value documents. Properties of particular luminescent phosphors used in a given value document are generally tightly guarded in order to maintain a high level of security. The presence of the luminescent phosphors allow for rapid validation of the value document when a number of parameters are interrogated.
To enhance security and difficulty in identification of particular luminescent phosphors, the value documents often include a plurality of regions that include different ink compositions that have different luminescent phosphors, and the validation techniques include detecting the presence of particular luminescent phosphors in distinct regions of the value document. However, over the past several years, counterfeiting has become an increasing problem and the ability of the counterfeiters to mimic the security features has increased substantially. Once sophisticated counterfeiters identify that a value document has a plurality of regions that include different inks, modern spectroscopic tools as well as public domain documents on emissive materials that include spectra can often be employed to identify specific luminescent phosphors. Once identified, the luminescent phosphors can be mimicked and used to create value documents that can pass high speed sorting tests.
The approach of going to more esoteric luminescent phosphors and the use of multiple materials often leads to higher usage rates, more complex production methods, higher costs, and lower material yields. Esoteric luminescent phosphors that can be produced in laboratory environments may prove challenging to scale to commercially required amounts.
Validation systems that involve detecting the decay time of a luminescent phosphor that is excited by a visible or ultraviolet light source in a stationary situation have been available for years and, recently, techniques have been developed that allow decay time constants (Tau) of luminescent phosphors to be measured at high speed in sorting machines. The decay time constants of luminescent phosphors can be controlled by the use of disturbing ions that impact decay times of the luminescent phosphors or by other effects such as concentration quenching. However, counterfeiters are still capable of identifying decay time parameters and mimicking the decay time parameters of specific luminescent phosphors once chemical components in the luminescent phosphors are identified.
Accordingly, it is desirable to provide security features on substrates of an article that can be employed to validate the article under conditions in which the substrate is either stationary or moving, such as in high speed sorting devices. It is also desirable to provide security features that are difficult to replicate, and that contain separate regions including different inks whose differences are difficult to detect. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.